Now the son [Ishmael], born to the slave woman, was born simply of their natural relationship; but the son [Isaac], born to the free woman, was born in fulfillment of a [divine] promise.

Now, brothers, you [Christians] are children of the promise [made to Abraham. See 3:29] just like Isaac, [Abraham's son] was.

But, just as it was then --- the son born in the ordinary way [i.e., Ishmael] persecuting the one born according to the Holy Spirit's [leading, i.e., Isaac] --- so it is now [i.e., Jews urging compliance with the requirements of the law of Moses persecuting those who refuse to return to dependance on that law for salvation. See 3:1-3].

What does the Scripture say about this matter? [It says, Gen. 21:10-12], "Get rid of the slave woman [Hagar], with her son [Ishmael]; for the son of the slave woman [Ishmael] will not inherit [what God promised to Abraham and his descendants], along with the son of the free woman [i.e., Isaac]."